r.los generates a raster output map in which the cells that are visible from a user-specified observer position are marked with the vertical angle (in degrees) required to see those cells (viewshed). A value of 0 is directly below the specified viewing position, 90 is due horizontal, and 180 is directly above the observer. The angle to the cell containing the viewing position is undefined and set to 180.
http://grass.fbk.eu/grass62/manuals/html62_user/r.los.html